The ability to produce finely atomized fluid sprays benefits many diverse applications including the manufacture of substrates for industry, the fueling of combustion systems, including the fueling of internal and external combustion engines, the formation of uniform-sized particles for the production of pharmaceutical products, the production of small particles for use as test standards and various applications in the electronics industry, in which thin-film deposition techniques are often employed to form resistors, capacitors and other components.
In general, the completeness and cleanliness of liquid fuel combustion depends upon the fuel/air ratio, the combustion chamber mechanical and aerodynamic design, the fuel type, the fuel injector design and the fuel droplet size distribution. A primary driver in combustion system design in recent years has been the reduction of combustion-generated emissions. This has applied across a broad range of applications, from residential heating equipment to automotive internal combustion engines to gas turbines to industrial and utility furnaces. The liquid fuel preparation method has a very significant impact on the resultant emissions, particularly emissions of carbon monoxide (CO), unburned hydrocarbons (HC) and soot. Thus in the drive to continuously reduce emissions from liquid fuel burning devices, there has been much effort directed at developing simple and cost-effective methods for achieving delivery of either vaporized fuel or very fine fuel droplets.
In any given liquid fuel combustion application, reduction of the droplet size can provide several benefits, including improved ignition characteristics, reduced droplet impingement on chamber walls, more rapid evaporation of the liquid droplets, reduced CO, HC and soot emissions, and the ability to operate with lower volatility (or heavier) liquid fuels. Though a fuel may be delivered to a combustion chamber in liquid droplet form, the liquid must evaporate before the fuel constituents can react with the oxygen in the combustion air. Large droplets evaporate slowly and may not have time to fully evaporate and react before exiting the combustion chamber, thereby resulting in higher emissions.
In particular, in the case of very small-scale combustion systems (less than, say, 10 kW heat release), the importance of achieving small droplet sizes is made all the more critical, especially for lower volatility fuels such as diesel or jet fuel. In addition, these small-scale systems require simple fuel delivery systems that do not use large amounts of power to prepare the fuel. Thus many of the conventional fuel delivery approaches (e.g. pressure atomization, twin-fluid or duplex atomization, ultrasonic atomization) cannot be applied to small-scale systems: flow rates are too high; droplets are too large, required supply pressures are too high or an additional atomizing fluid is required. Thus many small-scale combustion systems are limited to gaseous fuels.
A number of approaches to reduce the size of delivered fuel sprays have been proposed. For example, a combustion device wherein fuel is atomized by an ultrasonic atomizing device is disclosed in U.S. Pat. No. 5,127,822. According to this patent, atomizers have been proposed wherein fuel is supplied to a combustion chamber in fine droplets to accelerate vaporization of the fuel and reduce the time needed for steady combustion of the fuel.
U.S. Pat. No. 5,127,822 describes an arrangement wherein fuel is intended to be supplied at 5 cc/min and the fuel said to be atomized into droplets having a Sauter Mean Diameter (SMD) of 40 μm. Other atomizing techniques are proposed in U.S. Pat. Nos. 6,095,436 and 6,102,687. An ultrasonic atomizer intended for supplying fuel to an internal combustion engine is proposed in U.S. Pat. No. 4,986,248.
U.S. Pat. No. 4,013,396 proposes a fuel aerosolization apparatus wherein a hydrocarbon fuel (e.g., gasoline, fuel oil, kerosene, etc.) is to be dispensed into a condensation area to form an aerosol fuel said to exhibit relatively even sized droplets less than 1 μm in diameter. The aerosolized fuel is intended to be mixed with air to provide a desired air-fuel ratio and combusted in the combustion area of a burner. A heat exchanger is proposed to transfer heat from the combusted fuel to a heat-carrying medium such as air, gas, or liquid.
In U.S. Pat. No. 5,472,645, a fuel-vaporizing device is proposed to address certain problems associated with incomplete combustion of fuel aerosols in internal combustion engines. According to U.S. Pat. No. 5,472,645, because aerosol fuel droplets do not ignite and combust completely in internal combustion engines, unburned fuel residues are exhausted from the engine as pollutants such as hydrocarbons (HC), carbon monoxide (CO), and aldehydes with concomitant production of oxides of nitrogen (NOx). U.S. Pat. No. 5,472,645 proposes to improve combustion of aerosol fuels by breaking liquid fuel down into an air and fluid stream of vaporized or gas-phase elements. These elements are said to contain some non-vaporized aerosols of higher molecular weight hydrocarbons, with the lighter fuel components said to quickly evaporate to the gas phase, mix with air and fed to an internal combustion engine. The heavier fuel portions are said to be transformed into a gas-phase-vaporized state before they can exit a cyclone vortex device and enter the intake manifold of the engine.
U.S. Pat. No. 4,344,404 proposes an apparatus for supplying aerosolized fuel droplets mixed with air to an internal combustion engine or burner, the fuel droplets said to have sizes of 0.5 to 1.5 μm. The liquid fuel in aerosol form is mixed with air in an air-fuel ratio of about 18:1, with the goal of reducing the levels of CO, HC and NOx emissions from the engine.
Several patents disclose techniques for vaporizing a liquid. For example: commonly assigned U.S. Pat. Nos. 5,743,251 and 6,234,167 disclose aerosol generators which vaporize a liquid in a heated capillary tube; U.S. Pat. No. 6,155,268 issued to Takeuchi discloses liquid flavoring supplied by capillary action through a flow passage to a heater disposed on an end of the flow passage to vaporize the liquid flavoring; U.S. Pat. No. 5,870,525 issued to Young discloses that liquid from a reservoir can be fed through a supply wick by capillary action to a boiler wick in which the liquid is heated and boiled to a vapor; and U.S. Pat. No. 6,195,504 issued to Horie et al. discloses heating a liquid in a flow passage to produce a vapor.
U.S. Pat. No. 3,716,416 discloses a fuel-metering device intended for use in a fuel cell system. The fuel cell system is intended to be self-regulating, producing power at a predetermined level. The proposed fuel metering system includes a capillary flow control device for throttling the fuel flow in response to the power output of the fuel cell, rather than to provide improved fuel preparation for subsequent combustion. Instead, the fuel is intended to be fed to the fuel cell for conversion to H2. In a preferred embodiment, the capillary tubes are made of metal and the capillary itself is used as a resistor, which is in electrical contact with the power output of the fuel cell. Because the flow resistance of a vapor is greater than that of a liquid, the flow is throttled as the power output increases. The fuels suggested for use include any fluid that is easily transformed from a liquid to a vapor phase by applying heat and flows freely through a capillary. Vaporization appears to be achieved in the manner that vapor lock occurs in automotive engines.
U.S. Pat. No. 6,276,347 proposes a supercritical or near-supercritical atomizer and method for achieving atomization or vaporization of a liquid. The supercritical atomizer of U.S. Pat. No. 6,276,347 is said to enable the use of heavy fuels to fire small, light weight, low compression ratio, spark-ignition piston engines that typically burn gasoline. The atomizer is intended to create a spray of fine droplets from liquid, or liquid-like fuels, by moving the fuels toward their supercritical temperature and releasing the fuels into a region of lower pressure on the gas stability field in the phase diagram associated with the fuels, causing a fine atomization or vaporization of the fuel. Utility is disclosed for applications such as combustion engines, scientific equipment, chemical processing, waste disposal control, cleaning, etching, insect control, surface modification, humidification and vaporization.
To minimize decomposition, U.S. Pat. No. 6,276,347 proposes keeping the fuel below the supercritical temperature until passing the distal end of a restrictor for atomization. For certain applications, heating just the tip of the restrictor is desired to minimize the potential for chemical reactions or precipitations. This is said to reduce problems associated with impurities, reactants or materials in the fuel stream which otherwise tend to be driven out of solution, clogging lines and filters. Working at or near supercritical pressure suggests that the fuel supply system operate in the range of 300 to 800 psig. While the use of supercritical pressures and temperatures might reduce clogging of the atomizer, it appears to require the use of a relatively more expensive fuel pump, as well as fuel lines, fittings and the like that are capable of operating at these elevated pressures.